1. Field of the Invention
The present invention relates to a process for the preparation of (co)polychloroprene latex in water emulsion, in the presence of an emulsifying system which does not require the presence of surface-active agents consisting of carboxylic acids.
2. Description of the Related Art
(Co)polychloroprene latexes are produced with the classical method of emulsion polymerization, in the presence of emulsifying agents.
If the polymerization is carried out in an alkaline medium, the emulsifying agents usually comprise alkaline salts of rosin. But in some applications, such as formulations for adhesives with a solvent, the presence of rosin is not advisable in too high a quantity as this can cause separation of the phases in the presence of metal oxides.
If a polymerization in an acid medium is necessary, for example to copolymerize chloroprene with unsaturated carboxylic acids, it is not possible to use rosin owing to the pKa of the corresponding resinic acids. In this case resort must be made to other ionic surface-active agents, such as those containing sulphonate or sulphate groups.
Patent FR-A-2 333 818 describes a process for the production of concentrated latexes of polychloroprene which uses, for 100 parts of monomer, from 3 to 6 parts by weight of ionic emulsifying agents together with from 0.5 to 6 parts by weight of non-ionic emulsifying agents. This relatively high quantity of ionic emulsifying agent of the sulphate or sulphonate type has several disadvantages. In fact during polymerization the exothermic reaction is difficult to control and the heat which develops can exceed the capacity of the heat transfer of the polymerization plant. The higher the concentration of monomers in the aqueous phase, the more delicate the control of the reaction becomes. On the other hand in numerous applications it is advisable to have latexes with a high content of solids, preferably at least equal to 55%, as this creates fewer problems of storage, handling and transport.
The joint addition of a non-ionic surface-active agent enables, as is known at present, the emulsion to be stabilized and the gel point to be lowered. According to the above-mentioned document, the quantity of ionic emulsifying agent should not however be less than 3% of the polymerizable monomers. In addition, the use of high quantities of non-ionic products has negative effects on the reaction kinetics and on the control of the size distribution of the polymeric particles.
Another disadvantage of using very high quantities of ionic emulsifying agents of the sulphate or sulphonate type consists in the fact that it is difficult, if not impossible, to destabilize the resulting latex by cooling in order to isolate the polymer.
On the other hand the use of latexes as such (or in the presence of ionic surface-active agents), can be hindered in certain cases by a lack of colloidal stability in the presence of polyvalent metallic ions. In certain fields of application such as finishing, impregnation (for example of fibres), formulated water glues, it is preferable for the latex not to coagulate prematurely in the presence of metal ions, such as Ca.sup.++, Zn.sup.++, Al.sup.+++ or other ions which may be present in the formulations.
Patent FR-A-2 231 725 describes a process which enables the production of polychloroprene latexes which do not coagulate if electrolytes or other products present in compositions based on latex, for example zinc oxide, are added. According to the above process, the chloroprene is polymerized in emulsion without carboxylic soap in the presence of a polyvinyl alcohol. The latex obtained is not stable to storage owing to the hydrolysis of the colloid and in addition the polyvinyl alcohol reduces the resistance to water of the formulated products. This creates a problem for applications such as floor finishing.
Patent EP-A-457 642 describes a process which enables latexes to be obtained which are stable to storage and resistant to polyvalent metallic ions, characterized in that the emulsifying system comprises (a) at least one non-carboxylated ionic surface-active agent and (b) at least two non ionic surface-active agents which differ in the HLB value by 3 units or more. The latex obtained with the above process is stable to storage and resistant to metal ions, but does not have good chemical stability, i.e. it may be sensitive to various ingredients which are introduced during the formulation.